(1) Field of the Invention
This invention relates to the field of electrical circuit packages, and more specifically to packages for circuits designed to operate at radio frequency, microwave, millimeter wave frequencies, and up into the terahertz waveband. For the purposes of this specification such frequencies will be referred collectively as high frequencies.
(2) Description of the Art
Electrical circuits operating at high frequencies will, due to their nature, often radiate electromagnetic (EM) energy. Such radiation is often unwanted, and can cause problems if the radiation couples into nearby components or circuit board tracks etc, and can lead to unpredictable or unwanted circuit behaviour. For this reason, particularly sensitive parts of a circuit, or those components that are particularly prone to radiate, are frequently packaged in metal or otherwise electrically conductive packages. This can reduce the coupling between circuitry mounted in separate packages, and so alleviate the problem. However, this can sometimes lead to an increased EM coupling within an individual metallic package, leading to a particular sub-circuit mounted within a package to couple back to itself, or to other sub-circuits within the same package.
The problem gets worse as the frequency of operation gets higher, as the EM radiation wavelength will get correspondingly shorter, and hence be closer to a length at which the package itself will be resonant. Resonances can cause strong enhancements in field strengths, and this will tend to result in increased coupling between components, or circuit board tracks, and even between different elements of the same component within the package, which can lead to undesired circuit behaviour generally, and even oscillation if active circuits are involved.
A known solution is to coat the top and/or sides of the package with radiation absorbent material (RAM), or place blocks of RAM into the cavity. RAM is a material having the property that it tends not to reflect incident EM radiation, or allow all the radiation to pass through it. Instead, it is designed to absorb the radiation, effectively dissipating the energy as heat. Some types of these materials are quite thick, as to be effective they need to match the impedance of the radiated wave to the resistance of a dissipative material within the RAM. These are awkward to use in high frequency packages, particularly at the millimeter wave frequencies, due to the size of package required to accommodate the RAM, and the need to fabricate materials with the necessary material parameters.
Other types of RAM are much thinner. Emerson & Cuming Microwave Components manufacture a variety of sheet materials able to act as RAM. Some of these rely upon wavelength dependent effects, and so are inherently narrow band. Other types have a specific resistance that is graded throughout the material thickness to reduce reflections from the surface. These work over a broader bandwidth, but also tend to be thicker. When used in a cavity in which electronic components are mounted the absorption efficiency of these materials tends to reduce as the frequency of operation of the circuitry increases.
U.S. Pat. No. 6,054,766 discloses an electrical circuit package designed to address some of the problems described above. The package disclosed is made in whole or in part of silicon sheets, the sheets having a stated specific conductivity σ of between 1 and 10Ω−1 m−1. Traditionally, packages for high frequency circuits are often made from a solid piece of aluminium, with cavities milled out to hold the circuits themselves. Thus the disclosure of U.S. Pat. No. 6,054,766 will thus require the traditional packages to be abandoned, or, at best, modified so as to enable the silicon sheeting to be used as described therein.
Monolithic microwave Integrated Circuits (MMICs) operating at several tens of gigahertz have been packaged using the flip-chip technique, whereby the MMIC is flip-chip bonded to a lossy silicon substrate to suppress parasitic modes within the package. This is described in A Flip-Chip Packaged Coplanar 94 GHz Amplifier Module With Efficient Suppression of Parasitic Substrate Effects, Tessmann et al, IEE Microwave and Wireless Components Letters, Vol 14, No 4, April 2004. As the flip-chip techniques involves positioning the device upside down onto a substrate, once so bonded it is difficult to get access to the device for subsequent analysis, manipulation or modification, so the technique is not suitable for all situations. There are also problems associated with maintaining the designed electrical performance, such as the frequency characteristic, when the device is flipped over.
UK Patent Application No 0128208.6 describes a shielding means for electromagnetic interference, wherein a cavity employs a divider wall mounted on a lid, the wall effectively separating the cavity into smaller parts. The wall is positioned and shaped so that separate components within the cavity are effectively isolated from each other. This approach loses its effectiveness if sensitive components are mounted very close to radiating components.